So-called xe2x80x9cpositioning resinsxe2x80x9d must exhibit high dimensional stability and low movement throughout the range of operating conditions (especially thermal excursions) to which they are subjected. Stress, low durability, and insufficient strength to prevent relative movement between parts are primary causes of adhesive failure in many such applications, as in the manufacture of optical devices. Note might be made in this regard to a paper entitled xe2x80x9cAdvances in Light Curing Adhesives,xe2x80x9d Bachmann, A. G. (Proc. SPIE Vol. 4444, August 2001, pp 185-195).
In the present state of the art, the provision of resins of this kind normally focuses upon the glass transition temperature of the cured material as a measure of utility; it is common to use resins that are selected or formulated to have Tg values higher than necessary, to afford suitable tolerances. Reference might be made in this regard to an article by Tample-vich, T. W. and Moore, V. E., entitled: xe2x80x9cThe Significance of Glass Transition Temperature on Epoxy Resins for Fiber Optic Applicationsxe2x80x9d (Epoxy Technology, Inc. 1980). The satisfaction of a high Tg criterion (if possible at all, as a practical matter) often means that other desirable properties, such as tensile strength, lap shear, adhesive bond strength and modulus, and specific surface characteristics, must be compromised or sacrificed.
Thus, there is a need for formulations that are suitable for use as adhesives, potting compounds, and the like, which are of practical viscosity, which cure and endure without significant shrinkage, which are virtually immobile after reaction even when subjected to heat cycles of substantial temperature variation, and which may additionally exhibit other desirable properties as well.
The broad object of the present invention is to provide formulations that satisfy the foregoing needs.
It has now been found that the foregoing and related objects of the invention are attained by the provision of a fluid mixture capable of curing to a substantially nonshrinking and immobile solid mass (e.g., a positioning resin), comprising: about 10 to 50 percent, based upon the weight of the mixture, of a composition capable of reaction to form a solid, resinous matrix; and conversely, about 90 to 50 percent, based upon the weight of the mixture, of a solid filler comprised of spherical elements and short fibrous elements present in a spherical element:fibrous element weight ratio in the range 0.1 to 6:1, the filler elements being substantially nonreactive to the reactive composition and exhibiting good adhesion to the resinous matrix; in preferred embodiments, the mixture will comprise at least about 25 weight percent of the reactive composition and not more than about 75 weight percent of the filler combination. The composition will usually contain a free radical photoinitiator, in an amount sufficient to render it readily curable by exposure to actinic radiation; thermal initiators may however also be used, as appropriate, alone or in combination with one or more photoinitiators.
The spherical filler elements will usually constitute about 30 to 45 weight percent of the mixture described, and the fibrous elements may constitute about 10 to 80 weight percent thereof. Preferably, the fibrous elements will constitute about 20 to 30 weight percent of the mixture, and the spherical element: fibrous element ratio range will be about 1 to 3:1. The spherical and fibrous filler elements will generally be made of glass, for transparency (especially when photoinitiators are used) and low coefficient of thermal expansion (albeit ceramic, mineral, metal,and synthetic and natural resinous elements may be employed in certain instances), and they may or may not carry a size coating (e.g., an epoxy sizing, such as Owens Corning 731-EC milled fiber, or a silane sizing, such as Owens Coming 737BC milled fiber; Owens Corning 739DC milled fiber, which is an unsized product, may also be used, for example, to good effect). Indeed, a surprising aspect of the invention (to be described more fully below) is that, whereas at least certain size coatings cause very poor adhesion in formulations filled with fibers only, the same formulation can exhibit extraordinary adhesion when spherical filler elements are also included, in suitable amounts.
The spherical elements will preferably be hollow in most instances, with a distribution range of, for example, 5 to 20 microns and a mean size of 9 to 13 microns; the fibrous elements will preferably be short, small diameter milled fibers (albeit chopped fibers may be employed as well, generally to lesser advantage) with a screen size (hole diameter) parameter ranging from {fraction (1/16)} to {fraction (1/64)} inch. Typically, the sphere diameter will be 10 mils or smaller, and equal to or (preferably) less than the diameter of the fibers. The size, composition, and form of the spheres and fibers will generally be selected so as to afford good suspension stability in the mixture. The solid mass produced will most desirably have a glass transition temperature in excess of 100xc2x0 C.
Suitable solid glass spheres are available commercially from Potters Industries Inc. under its SPHERIGLASS trademark, and suitable hollow glass spheres are available from the same company under its SPHERICEL trademark. Ceramic microspheres that may also be suitable for use are available from Zeelan Industries under its ZEEOSPHERES W-210 designation.
Although a wide variety of polymerizable compositions can be employed to produce the resinous matrix, the composition will desirably comprise a substantial amount of epoxy, especially multifunctional epoxy, resin. More particularly, the photopolymerizable composition may advantageously comprise, based upon the total weight thereof, about 5 to 45 percent of an epoxy resin, about 94 to 55 percent of a copolymerizable material (i.e., monomer and/or oligomer), and about 1 to 10 percent of a free radical photoinitiator. The composition may, more specifically, comprise about 15 to 30 weight percent of epoxy resin, about 40 to 60 weight percent of at least one comonomer, about 20 to 35 weight percent of an oligomer that is reactive with the epoxy resin and the comonomer, and about 3 to 5 weight percent of the photoinitiator; in many instances it will, most advantageously, be devoid of any catalytic cationic ingredient.
The xe2x80x9cat least onexe2x80x9d comonomer will desirably be N,N-dimethylacrylamide, in which case the composition will beneficially contain about 20 weight percent of epoxy resin, about 50 weight percent of N,N-dimethylacrylamide, and about 25 weight percent of a cellulosic oligomer. More broadly, however, the comonomer compound will advantageously be selected from the group consisting of vinyl and (meth)acrylic monomers containing acrylamide or amide functionality, or an hydroxyl group. Thus, additional specific members of the comonomer group include, for example, n-vinyl 2-pyrrolidone, n-vinylcaprolactam, acryloyl morpholine, N-(n-butoxymethyl) acrylamide, N-isopropyl acrylamide, N-3-dimethylaminopropyl methacrylamide, glycerol 1,3-diglycerolate diacrylate, 4-hydroxybutyl acrylate, 2-hydroxyethyl methacrylate, and (meth)acrylic acid; this group may also include acrylated polyols and vinyl polyols, albeit such compounds may be of either monomeric or oligomeric character.
When the comonomer is or includes a compound containing the amide or acrylamide functionality, that compound will usually be present in an amount not in excess of 80 weight percent of the polymerizable composition, and preferably the amount will be at least about 50 weight percent. When the comonomer is or includes a compound containing the hydroxyl group, that compound will usually be present in an amount not in excess of about 70 weight percent, and preferably the amount will be at least about 20 weight percent.